It is well known to use lasers to cut, score, or engrave surfaces of different materials. A laser produces a monochromatic beam of light in a known manner, and a focusing lens is used to reduce the diameter of the beam and adjust the depth of field of the focused beam. Before initiating a cutting operation, the laser power supply and feedrate of the laser head are adjusted in accordance with the material being cut. For example, to cut metals, the laser power is set higher than if softer materials such as wood are to be cut. In addition, the distance between the surface on the workpiece to be cut and the focusing lens is initially set. By setting that distance to the focal length, the smallest cutting beam is achieved for cutting along fine or thin lines. The distance between the laser head and the surface to be cut can be set to less than the focal length, thereby producing a larger cutting beam for cutting wider lines in the surface.
If the surface on the workpiece is a planar surface, once the desired distance between the surface and focusing lens is set, it does not change and therefore, does not have to be adjusted or corrected during the laser cutting process. In contrast, if the surface on the workpiece to be cut is curved, the distance between the laser head and the workpiece must be continuously adjusted as the laser head is moved over the curved surface. For example, as shown in U.S. Pat. No. 5,538,056, ultrasonic positioning sensors are associated with a laser head to maintain a desired distance between the laser head and the workpiece as the laser head moves over the cylindrical surface of the workpiece. In other applications, for example, as disclosed in U.S. Pat. No. 4,948,940, optical, inductive, or capacitive-type sensors are used to determine the distance between the laser head and the surface to be cut, thereby permitting the position of the laser head to be continuously adjusted as it moves over the curved surface. While such systems are effective, they require relatively expensive sensors and control devices. Thus, they add substantially to the cost of the laser engraving machine.
Therefore, there is a need to provide a laser cutting or engraving machine which has an improved device for controlling the position of the laser head with respect to the curved surface being cut.